This proposal is a continuation of previously funded research on portion size, which clearly established that large portions have a powerful effect on food consumption, and showed more recently that changes in portion size can be used to reduce energy intake in the short term. It is now essential to translate these findings to the management of body weight. The primary aim of this research is to determine the efficacy of two promising portion-control strategies to achieve sustainable dietary and behavior changes and to promote weight loss and maintenance. We propose a randomized controlled trial to test two different approaches to managing portion sizes: one that gives individuals skills and tools to help them make appropriate portion choices, and another that focuses on consumption of pre-portioned foods in order to limit exposure to large portions in the personal food environment. During a 6-month intervention phase, 300 overweight and obese women and men will meet weekly with dietitians for individual instruction; this will be followed by 12 months of monthly group meetings to promote the maintenance of behavior changes and weight loss. Participants will be randomly assigned to one of three equally intensive intervention programs. The portion-size instruction group will be given training in selecting portions and in the use of portion-control tools such as measuring implements, photographs, and charts. The instruction will include an innovative approach based on previous research from this grant, which shows that consuming larger portions of foods that are low in energy density (such as vegetables and water- rich foods) enhances satiety and reduces energy intake. A second group of participants will be provided with pre-portioned foods (such as frozen entries, fruit and vegetables, and yogurt) for two meals and one snack daily during the intervention phase, in addition to receiving basic nutritional instruction for weight loss. The third group will receive only the basic nutritional program. It is hypothesized that the two portion-control interventions will result in increased weight loss during the intervention phase, but that the maintenance of weight loss will depend on sustained changes in the acquisition and application of portion size knowledge. An innovative aspect of this research is that it will not only evaluate the usual outcomes associated with weight loss, but will also use state-of-the-art methods to assess the impact of the interventions on the understanding and implementation of portion-control strategies. Thus, the second aim of the research is to determine the effects of the programs on knowledge and consumption of appropriate portions, as assessed by innovative software and multiple-pass dietary recalls. The third aim is to conduct exploratory analyses of individual factors that may influence the response to the portion-control interventions, such as psychosocial indicators, blood biomarkers, and measures of adherence to the interventions. The expected outcome of the project is that it will lead to the development of specific, evidence-based strategies to help control portion sizes in order to manage weight.